Since your AC switch is on the panel, I think you need to check the AC breaker first.
Next step is outside for the reset button for the high limit switch. They should be something like this.
After that you will need a meter to check for voltage at the bottom of the left thermostat (water heater cold). If no voltage your thermostat could be bad. If voltage, your heater element is likely shot.
Lou and Laura with Bella - German Short Hair Pointer
2008 GMC Sierra 2500HD Crewcab SB Allison Duramax
2010 Flagstaff 8526RLWS - Superglide 3300
HAM CALLSIGN - KC3FFW